The present invention concerns methods of synthesizing photo-curable preceramic polymers, and more particularly, poly(ethynyl)carbosilane.
Silicon carbide (SiC) is one of several advanced ceramic materials which are currently receiving considerable attention as electronic materials, as potential replacements for metals in engines, and for a variety of other applications where high strength, combined with low density and resistance to oxidation, corrosion and thermal degradation at temperatures in excess of 1000° C. are required. Unfortunately, these extremely hard, non-melting ceramics are difficult to process by conventional forming, machining, or spinning applications rendering their use for many of these potential applications problematic. In particular, the production of thin films by solution casting, continuous fiber by solution or melt spinning, a SiC matrix composite by liquid phase infiltration, or a monolithic object using a precursor-based binder/sintering aid, all require a source of SiC which is suitable for solution or melt processing and which possesses certain requisite physical and chemical properties which are generally characteristic of polymeric materials.
Polymeric precursors to ceramics such as SiC afford a potential solution to this problem as they would allow the use of conventional processing operations prior to conversion to ceramic. A ceramic precursor should be soluble in organic solvents, moldable or spinnable, crosslinkable, and give pure ceramic product in high yield on pyrolysis. Unfortunately, it is difficult to achieve all these goals simultaneously. Currently available SiC precursor systems are lacking in one or more of these areas. Problems have been encountered in efforts to employ the existing polysilane and polycarbosilane precursors to SiC for preparation of SiC fiber and monolithic ceramic objects. All of these precursors have C/Si ratios considerably greater than one, and undergo a complex series of ill-defined thermal decomposition reactions which generally lead to incorporation of excess carbon. The existence of even small amounts of carbon at the grain boundaries within SiC ceramics has been found to have a detrimental effect on the strength of the ceramic, contributing to the relatively low room-temperature tensile strengths typically observed for precursor-derived SiC fibers.
Efforts to develop polymeric precursors to SiC have focused largely on two types of polymers, polysilanes, which have a Si—Si backbone, and polycarbosilanes, in which the polymer backbone is [—Si—C—].sub.n. The polysilanes all suffer from problems due to insolubility, infusibility and/or excess carbon incorporation. Certain of the polycarbosilanes have more suitable physical properties for processing; however, in general, these also contain a higher-than-1:1 C:Si ratio and incorporate excess carbon on pyrolysis.
In the case of the polycarbosilanes, high molecular weight linear polymers of the type [R2SiCH2]n, where R is H and/or hydrocarbon groups, have been prepared by ring-opening-polymerization reactions starting from cyclic disilacyclobutanes using chloroplatinic acid and related catalyst systems; however, such linear polycarbosilanes generally exhibit low yields of ceramic product on pyrolysis due to chain “unzipping” reactions. For example, studies of high molecular weight [Me2SiCH2]n polymers have indicated virtually complete volatilization on pyrolysis under an inert atmosphere to 1000° C.
Use of propargyl groups (HC≡CCH2—), such as propargyl chloride (HC≡CCH2Cl), propargyl bromide (HC≡CCH2Br), propargyl alcohol (HC≡CCH2OH), propargyl magnesium chloride (HC≡CCH2MgCl), propargyl calcium chloride (HC≡CCH2CaCl), propargyl amine (HC≡CCH2NH2), and other propargyl-containing species introduces the photo-curable (cross-linkable) triple-bonded carbon linkages into the pre-ceramic polymer.
U.S. Pat. No. 5,153,295 teaches the use of preceramic polymers with an Si—C backbone structure, such as allylhydridopolycarbosilane (AHPCS), formed from the Grignard coupling reaction of a halomethylcarbosilane followed by reduction using a metal hydride in which either a UV cross-linkable ethynyl (i.e. acetylide) or propargy] group has been introduced into the polymer. A key feature of these polymers is that substantially all of the linkages between the Si—C units are “head-to-tail”, i.e., they are Si to C. The polycarbosilane “SiH2CH2” has a carbon to silicon ratio of 1 to 1 and where substantially all of the substituents on the polymer backbone are hydrogen. These polymers have the advantage that it is only necessary to lose hydrogen during pyrolysis, thus ceramic yields of over 90% are possible, in principle. The extensive Si—H functionality allows facile crosslinking and the 1 to 1 carbon to silicon ratio and avoids the incorporation of excess carbon in the SiC products that are ultimately formed. The synthetic procedure employed to make them allows facile modification of the polymer, such as by introduction of small amounts of pendant vinyl groups, prior to reduction. The resulting vinyl-substituted “SiH2CH2” polymer has been found to have improved crosslinking properties and higher ceramic yield.
A pre-ceramic polymer has been prepared by a thermally induced methylene insertion reaction of polydimethylsilane. The resulting polymer is only approximately represented by the formula [SiHMeCH2]n, as significant amounts of unreacted (SiMe2)n units, complex rearrangements, and branching are observed. In addition to the carbosilane “units”, large amounts of Si—Si bonding remains in the “backbone” of the polymer. This polymer disadvantageously contains twice the stoichiometric amount of carbon for SiC formation. The excess carbon must be eliminated through pyrolytic processes that are by no means quantitative. Despite the shortcomings, this polymer has been employed to prepare “SiC” fiber. However, it must be treated with various crosslinking agents prior to pyrolysis which introduce contaminants. This results in a final ceramic product that contains significant amounts of excess carbon and silica which greatly degrade the high temperature performance of the fiber.
SiC precursors, predominately linear polycarbo-silanes, have been prepared via potassium dechlorination of chloro-chloromethyl-dimethylsilane. The resulting polymers have not been fully characterized, but probably contain significant numbers of Si—S1 and CH2—CH2 groups in the polymer backbone. The alkali metal dechlorination process used in the synthesis of such materials does not exhibit the selective head-tail coupling found with Grignard coupling. The pendant methyl groups in such materials also lead to the incorporation of excess carbon into the system. In several polymer systems mixtures containing vinylchlorosilanes (such as CH2≡CH—Si(Me)Cl2) and Me2SiCl2 are coupled by dechlorination with potassium in tetrahydro-furan. U.S. Pat. No. 4,414,403 and U.S. Pat. No. 4,472,591 both teach this method. The “backbone” of the resulting polymers consists of a combination of Si—Si and Si—CH2CH(—Si)2 units. Later versions of this polymer Me(H)SiCl2 in addition to the Me2SiCl2 and are subjected to a sodium-hydrocarbon dechlorination process which does not attack vinyl groups. The resulting polymer consists of a predominately linear, Si—Si “backbone” bearing pendant methyl groups, with some Si—H and Si—CH≡CH2 functionality to allow crosslinking on pyrolysis.
None of these precursors derived using vinylchlorosilanes are similar to those of the process in that having predominantly Si—Si bonded “backbones”, they are essentially polysilanes, not polycarbosilanes. In addition, the carbon in these polymers is primarily in the form of pendant methyl functionality and is present in considerable excess of the desirable 1 to 1 ratio with silicon. The ceramic products obtained from these polymers are known to contain considerable amounts of excess carbon.
Polymeric precursors to SiC have been obtained by redistribution reactions of methyl-chloro-disilane (Me6-xClxSi2, x=2–4) mixtures, catalyzed by tetraalkyl-phosphonium halides which U.S. Pat. No. 4,310,481, U.S. Pat. No. 4,310,482 and U.S. Pat. No. 4,472,591 teach. In a typical preparation, elemental analysis of the polymer was employed to suggest the approximate formula [Si(Me)1.15(H)0.25]n, with n averaging about 20. The structures of the polymers involve what is reported to be a complex arrangement of fused polysilane rings with methyl substitution and a polysilane backbone.
The formation of carbosilane polymers with pendent methyl groups has been prepared as by-products of the “reverse-Grignard” reaction of chloromethyl-dichloro-methylsilane. The chief purpose of this work was the preparation of carbosilane rings and the polymeric byproduct was not characterized in detail nor was its use as a SiC precursor suggested. Studies of this material indicate that it has an unacceptably low ceramic yield on pyrolysis. These polymers contain twice the required amount carbon necessary for stoichiometric silicon carbide and their use as SiC precursors was not suggested. Moreover, the starting material, chloromethyl-dichloro-methylsilane, contains only two sites on the Si atom for chain growth and therefore cannot yield a structure which contains tbd.SiCH2— chain units. On this basis, the structure of the polymer obtained, as well as its physical properties and pyrolysis characteristics, is not optimal for use as an SiC precursor.
U.S. Pat. No. 4,631,179 teaches a polymer which is a product of the ring-opening polymerization of (SiH2CH2)2 also has the nominal composition “SiH2CH2”. However, the actual structure of this polymer is reported to be a linear polycarbosilane which presumably has only [SiH2CH2] as the internal chain segments. The (SiH2CH2)2 monomer used by Smith is difficult and expensive to prepare and not generally available.